Identity Model

{
  userId: ObjectId                    // Reference to User
  provider: IdentityProvider         // LOCAL, GOOGLE, GITHUB, etc.
  providerAccountId: string          // Unique ID from provider
  email?: string                     // Email for this identity
  emailVerified?: boolean            // Verification status
  passwordHash?: string              // For LOCAL only
  salt?: string                      // For LOCAL only
  providerData?: {                   // OAuth provider data
    picture?: string
    locale?: string
    [key: string]: any
  }
  createdAt: Date
  updatedAt: Date
}

enum IdentityProvider {
  LOCAL = 'local',
  GOOGLE = 'google',
  GITHUB = 'github',
  MICROSOFT = 'microsoft',
}

const identity = new Identity({
  userId: user._id,
  provider: IdentityProvider.LOCAL,
  providerAccountId: user._id.toString(),
  email: 'john@example.com',
  emailVerified: true,
  passwordHash: hashedPassword,
  salt: salt.toString('base64')
})
await identity.save()

const identity = new Identity({
  userId: user._id,
  provider: IdentityProvider.GOOGLE,
  providerAccountId: googleProfile.sub,
  email: googleProfile.email,
  emailVerified: true,
  providerData: {
    picture: googleProfile.picture,
    locale: googleProfile.locale
  }
})
await identity.save()

// By email and provider
const identity = await Identity.findOne({
  email: 'john@example.com',
  provider: IdentityProvider.LOCAL
})
 
// All identities for a user
const identities = await Identity.find({ userId: user._id })

// User has both email and Google login
const identities = await Identity.find({ userId: user._id })
// Returns:
// [
//   { provider: 'local', email: 'john@example.com' },
//   { provider: 'google', email: 'john@gmail.com' }
// ]